1. Field of the Invention
This invention relates to heat exchangers and, more particularly, to a tube support system for U-shaped tubes in heat exchangers of the shell and tube type.
2. Description of the Prior Art
Heat exchangers of the shell and tube type generally include a cylindrical outer shell and a bundle of inner tubes transiting the length of the shell. Commonly, each of the tubes is formed in the shape of an elongated U with the U-bend portion position at one end of the shell and the straight portions or legs terminated in a perforated tube sheet at the opposite end of the shell. The tubes are maintained in a parallel relationship and restrained against excessive vibration by perforated, tube support plates fixed, at spaced intervals along the length of the shell, to a frame assembly attached to the tube sheet. First and second fluids, between which heat is to be exchanged, are passed through the tubes and through the shell region surrounding the tubes, respectively. Isolation between the fluids is provided by confining the first fluid within the tubes and by confining the second fluid within the shell region transited by the tubes.
Construction of heat exchangers in the manner described is advantageous in that it facilitates easy removal of tube bundles for maintenance and inspection, since the tubes are only fixedly attached at one end of the shell. Such construction, however, must allow for thermal expansion of the tubes relative to the fixed support plates and also relative to each other. It is known to make the perforations in the support plates oversized to allow relative movement between the tubes and the plates, but oversizing alone has proven to be inadequate in large heat exchangers operating at high temperatures. In such heat exchangers the tubes tend to bind in the support plate closest to the U-bends and tube or support plate failure may result. This binding is caused largely by two types of tube thermal expansion. The first of these is differential longitudinal expansion of the legs of individual tubes relative to each other. This type of expansion applies not only longitudinal forces tending to lengthen the individual tubes, but also transverse forces tending to move the tube legs out of the parallel relationship maintained by the support plates. Such movement is restricted by the support plate closest to the U-bends and the tube legs bind in this plate. The second type of thermal expansion is transverse expansion of the U-bends themselves. This type of expansion applies forces transverse to the leg axes tending to increase the separation between the tube legs. Since increased separation is also restricted by the support plate closest to the U-bends, the expansion of the U-bends also causes binding in this support plate.
The present invention provides support means for a tube bundle which overcomes the aforementioned binding problem and allows both differential longitudinal expansion of tube legs and transverse expansion of U-bends, without sacrificing the vibration inhibiting effect of support plates. Support means is also provided to allow expansion of members to which the support plates are fixed, relative to the shell. Additionally, support of a tube bundle in accordance with the present invention does not require any connection of the bundle to the shell which would prevent easy removal of the bundle from the shell.